In positioning systems based on satellite positioning, a positioning receiver attempts to receive signals from at least four satellites in order to determine the position of the positioning receiver as well as the time data. Some examples of such satellite positioning systems to be mentioned include the GPS system (Global Positioning System), the GLONASS (GLObal NAvigation Satellite System) as well as the European Galileo system under development. For example the GPS system comprises a plurality of satellites orbiting the globe according to predetermined orbits. These satellites transmit orbit data, on the basis of which the position of the satellite can be determined at each moment of time, provided that the exact time data used in the satellite positioning system is known in the positioning receiver. In the GPS system, the satellites transmit a spread spectrum signal modulated with a code which is individual for each satellite. Thus, the positioning receiver can distinguish between signals transmitted by different satellites by using a reference code corresponding to a satellite code generated locally in the positioning receiver or stored in the positioning receiver.
Each operating satellite of the GPS system transmits a so-called L1 signal at the carrier frequency of 1575.42 MHz. This frequency is also indicated with 154f0, where f0=10.23 MHz. Furthermore, the satellites transmit a L2 signal at a carrier frequency of 1227.6 MHz, i.e. 120f0. In the satellite, these signals are modulated with at least one pseudo random sequence. This pseudo random sequence is different for each satellite. As a result of the modulation, a code-modulated wideband signal is generated. This modulation technique allows the receiver to distinguish between the signals transmitted by different satellites, although the carrier frequencies used in the transmission are substantially the same. This modulation technique is called code division multiple access (CDMA). In each satellite, for modulating the L1 signal, the pseudo random sequence used is e.g. a so-called C/A code (Coarse/Acquisition code), which is a code from the family of the Gold codes. Each GPS satellite transmits a signal by using an individual C/A code. The codes are formed as a modulo-2 sum of two 1023-bit binary sequences. The first binary sequence G1 is formed with the polynomial X10+X3+1, and the second binary sequence G2 is formed by delaying the polynomial X10+X9+X8+X6+X3+X2+1 in such a way that the delay is different for each satellite. This arrangement makes it possible to generate different C/A codes by using identical code generators. The C/A codes are thus binary codes, chipping rate in the GPS system being 1.023 Mchips/s. The C/A code comprises 1023 chips, wherein the iteration time (epoch) of the code is 1 ms. The carrier of the L1 signal is further modulated by navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the “health”, orbit, time data of the satellite, etc.
In order to detect the satellite signals and to identify the satellites, the receiver must perform acquisition, whereby the receiver searches for the signal of each satellite at the time and attempts to be synchronized and locked to this signal so that the information transmitted with the signal can be received and demodulated.
The positioning receiver must perform the acquisition e.g. when the receiver is turned on and also in a situation in which the receiver has not been capable of receiving the signal of any satellite for a long time. Such a situation can easily occur e.g. in portable devices, because the device is moving and the antenna of the device is not always in an optimal position in relation to the satellites, which impairs the strength of the signal coming in the receiver. In portable devices, the aim is also to reduce the power consumption to a minimum. Thus, for example, a positioning receiver arranged in connection with a wireless communication device is not necessarily kept continuously in operation but primarily when there is a need to perform positioning.
The above-mentioned acquisition and frequency control process must be performed for each satellite signal received in the receiver. Some receivers may comprise several receiving channels, wherein an attempt is made on each receiving channel to acquire the signal of one satellite at a time and to find out the information transmitted by this satellite.
After the acquisition, the positioning receiver attempts to keep synchronized with, i.e., to track the satellite signal. For the acquisition, correlators are normally used for generating signals which are used, for example, to find the correct code phase. The satellite signal received in the receiver is sampled, and the samples are led to the correlators. In receivers of prior art, the sampling rate is typically determined according to the chips in the satellite signal so that the sampling rate is normally twice the chipping rate. This means that two samples are taken of each chip. Applied into the GPS system, this means that about 2 million samples are taken per second. In practice, such a sampling rate is normally sufficient for signal acquisition, but this sampling rate is not necessarily sufficient for tracking, particularly under conditions of multipath propagation, i.e. the satellite signals arrive at the receiver along various routes.
By the selection of the sampling rate, it is possible to affect, for example, the manufacturing costs and the power consumption of the receiver. Normally, a higher sampling rate also involves higher manufacturing costs as well as a higher power consumption, which is due, for example, to the fact that the number of correlators used for the acquisition should also be increased when the sampling rate is increased. A larger number of correlators also requires more circuit board area, which, in turn, increases the power consumption.
In positioning receivers of prior art, the sampling frequency is used for both the acquisition and the tracking. Thus, the sampling frequency is a compromise determined by various properties. Furthermore, in receivers which are intended for receiving signals from the satellites or other positioning stations of more than one positioning systems, there may be a need to use a different sampling rate in the different systems. Thus, when applying the arrangements of prior art, separate receiving channels and sampling means must be provided for the different systems, which makes the implementation of the receiver more complex.